This invention relates to an optical alignment structure for coupling optical radiation to an optical receiving device and, more particularly, to such a structure especially designed to couple the output of a laser diode to an optical fiber.
The light (optical radiation) emerging from a typical laser diode is highly divergent and elliptical. Some form of coupling optics, such as a spherical lens, graded index lens or a collimating/focusing lens set, is needed to match this light output to an optical receiving device, such as an optical fiber. This alignment becomes quite critical if it is desired to maximize coupling. For example, if the optical radiation from the laser diode is visible radiation having a wavelength of 670 nm, coupling of the same to a single mode optical fiber requires alignment precision within about .+-.0.5 microns. To prevent performance degradation over time, it is important that the coupling structure maintain this level of alignment throughout the lifetime of the laser diode/optical fiber combination.
Reasonably attained mechanical tolerances of the various components making up such a combination are such that the required level of precision generally cannot be obtained with the simple assembly of parts. Thus to compensate for part tolerances, some provision for precision alignment typically is provided. Prior coupling optics arrangements have often had fixed optics, and the diode and fiber have been moved in some combination in these prior arrangements until alignment is attained. Clearance must be allowed, of course, around any component that is adjusted, and some type of potting compound often is used to hold one or more components in alignment. Anomalous dimensional changes over a period of time are not uncommon where potting compounds are used, believed to be caused by shrinkage of the potting compounds. Moreover, thermal sinking of the parts, particularly when a laser diode is used as a source, is compromised if the parts are potted in place.